Heretofore, .alpha.-naphthol has been produced by a method which includes sulfonating naphthalene and subsequently subjecting the sulfonation product to alkali fusion. According to this method, however, .beta.-naphthol is produced as by-product, which is recovered with the .alpha.-naphthol. Commercially, .alpha.-naphthol of high purity cannot be obtained by this method without difficulty. Recently, there has been proposed the so-called oxidation and dehydrogenation process as a method for the commercial production of .alpha.-naphthol of high purity. This oxidation and dehydrogenation process effects the production of .alpha.-naphthol by the steps of oxidizing tetralin to .alpha.tetralone in the presence of a catalyst and subsequently dehydrogenating the .alpha.-tetralone in the presence of a catalyst. This oxidation and dehydrogenation process, however, has the disadvantage that in the oxidation of tetralin performed in the presence of a catalyst, .alpha.tetralol occurs as a by-product and the amount of this by-product eventually affects the yield of .alpha.-naphthol. To be more specific, when tetralin is subjected to oxidation in the presence of a catalyst, the reaction produces tetralin hydroperoxide (hereinafter referred to as "HPO") as the intermediate, which immediately converts itself into .alpha.-tetralone and .alpha.-tetralol. The .alpha.-tetralone ane .alpha.-tetralol have close boiling points and, therefore, cannot be separated from each other without difficulty. In the dehydrogenation of .alpha.-tetralone in the presence of a catalyst in this process, therefore, it is actually the mixture of .alpha.-tetralone and .alpha.-tetralol that undergoes the treatment for dehydrogenation. In the dehydrogenation, .alpha.-tetralol is dehydrogenated to 1,2-dihydronaphthalene and this 1,2-dihydronaphthalene undergoes further dehydrogenation to naphthalene. It follows that in the oxidation and dehydrogenation process, the amount of naphthalene increases and the yield of .alpha.-naphthol consequently decreases in proportion as the .alpha.-tetralol content increases in the mixture of .alpha.-tetralone and .alpha.-tetralol. The reaction mechanism which is involved in the oxidation and dehydrogenation process is indicated below. ##STR1##
It has been customary in this oxidation and dehydrogenation process to resort to extensive use as a catalyst a metal salt of an organic compound such as iron naphthenate or cobalt naphthenate. When such an organic metal salt is used as the catalyst, however, since the oxidation of tetralin is carried out at temperatures of 120.degree. to 130.degree. C., the HPO occurring as the intermediate in this reaction readily undergoes conversion into .alpha.-tetralol. Further at this temperature level, part of the .alpha.-tetralol thus produced undergoes dehydration.
In view of the above circumstances, attempts have been made to improve the catalyst in the oxidation of tetralin with a view toward lowering the .alpha.-tetralol content in admixture with .alpha.-tetralone.
U.S. Pat. No. 2,462,103 by Johnson et al. discloses the process comprising controllably oxidizing tetralin with air or oxygen in the presence or absence of catalyst in such manner as to form a maximum proportion of HPO and decomposing catalytically the formed HPO with an aqueous solution of a copper salt of a mineral acid.
This process has, however, an unsatisfactory results because the percentage of decomposition of HPO is not more than 90% and the highest ratio of .alpha.-tetralone/ .alpha.-tetralol obtained by the decomposition, namely the selectivity to .alpha.-tetralone over .alpha.-tetralol, is only the order of 15/1.
The article by Kharash et al., J. Org. Chem., vol. 15, pages 763-774(1950), discloses a process of decomposing HPO in the presence of ferrous ammonium sulfate in a nonaqueous solution, ethanol. This process of Kharash et al. disappointingly results in the low ratio of .alpha.-tetralone to .alpha.-tetralol of at most about 10/1 and in the formation of an unwanted by-product, .alpha.-alkoxynaphthalene, besides .alpha.-tetralol.